It is a goal of the tobacco industry to produce cigarettes that have a reduced tendency to start fires. Such cigarettes are already subject to legal regulations in various countries and regions, for example the USA, Canada, European Union or Australia. In order to determine whether a cigarette has a reduced tendency to start fires, a test described in ISO 12863:2010 or ASTM E2187 is used.
In the tests, a smoldering cigarette is placed on a defined substrate, for example 10 layers of Whatman™ No. 2 filter paper, and it is observed whether the cigarette self-extinguishes before the entire visible tobacco rod has smoldered away. In many cases the legal regulations demand that of 40 tested cigarettes, at least 30 have to self-extinguish.
One way of accomplishing self-extinguishing of a cigarette in this test consists in applying a composition that reduces the diffusion capacity of the cigarette paper onto areas of the cigarette paper. The functionality of these areas relies on the fact that they prevent the access of oxygen to the glowing cone of the cigarette and thereby lead to self-extinguishing of the cigarette. As the access of oxygen during smoldering is primarily determined by the difference in concentration between the interior of the cigarette and the surroundings, hence by diffusion, it is important to select the diffusion capacity of these areas to be sufficiently low.
The measurement of the diffusion capacity of such areas can be carried out with an appropriate measuring device, such as the Diffusivity Tester (Borgwaldt A50) manufactured by Borgwaldt KC GmbH of Hamburg, Germany, in accordance with CORESTA Recommended Method No. 77. The diffusion capacity thereof describes a gas transport through the cigarette paper driven by a concentration difference. It thus indicates the gas volume passing through the paper per unit time, per unit area and per concentration difference, and hence has the unit cm3/(cm2 s)=cm/s.
The diffusion capacity of the areas applied to the cigarette paper required for self-extinguishing of the cigarette depends not only on the properties of the cigarette, but also to a substantial extent on the tobacco blend in the cigarette and the geometry of the cigarette. It is known, for example, that a high proportion of coarse tobacco particles in the tobacco blend, so called stems, makes self-extinguishing less likely, while a high proportion of cut lamina or expanded tobacco is beneficial to self-extinguishing. It is also known that in general, cigarettes with smaller diameter self-extinguish more easily and a longer length for the tobacco rod is also beneficial for self-extinguishing for the simple reason that, for a given design of the areas, more areas treated with the composition are located on the tobacco rod of the cigarette than on a short tobacco rod.
The treated areas of the cigarette paper could simply be provided with a very low diffusion capacity and thereby achieve self-extinguishing substantially independently of the tobacco blend or the overall cigarette construction. This method, however, has the disadvantage that not only the diffusion of oxygen into the cigarette, but also the diffusion of carbon monoxide out of the cigarette, generated during smoldering and puffing in the tobacco rod, is prevented by the areas. Thus, the areas cause an increase in the carbon monoxide content in the smoke that is undesirable because of the usual legal provisions related to the maximum carbon monoxide content in the smoke. In addition, too low a diffusion capacity leads to frequent self-extinguishing of the cigarette during normal smoking, which reduces the smoker's acceptance of such cigarettes.
In the prior art it is known to provide the areas as bands in cross-direction of the cigarette paper so that they are located in circumferential direction on a cigarette manufactured from this paper. It is also known that a minimum width of the bands of 4 mm is required in many cases in order to obtain self-extinguishing at all. In practice, however, it frequently turns out that in order to comply with legal requirements, 6 mm wide bands, printed over their entire area, are typically necessary on the cigarette paper. The distance between the bands, however, typically results from the length of the tobacco rod of the cigarette, since a frequent legal requirement is that at least two bands have to be present on the tobacco rod. Other shapes for the areas, for example, 6 or 7 mm wide bands, divided by a 1 mm wide slit are also known. In principle, the areas can have any arbitrary shape as long as it is compatible with the process for applying the composition and a sufficient self-extinguishing rate can be ensured.
From the prior art, many compositions are known that can be applied in areas on the paper. Frequently, these are aqueous compositions that comprise at least one film-forming material. This film-forming material forms a superficial film during drying of the paper after application and thereby seals the pores of the paper and hence reduces the diffusion capacity. Other materials that enter into the pores instead are also known. In some cases, the composition also contains pigments. These pigments can provide a color to the areas, but in many cases they are white in order to equalize the opacity and the whiteness of the treated areas and the untreated paper and therefore make them barely visible.
Application of the composition to the paper can be carried out by any possible application method in the prior art; printing processes or spraying have proved to be of value. Application can be carried out after the production of the untreated paper or during paper production in the paper machine.
The manufacturer of cigarette papers for self-extinguishing cigarettes is thus required to provide a paper the diffusion capacity of which is adapted to the entire cigarette construction such that the legal requirements are complied with, but that the diffusion capacity is not unnecessarily low. Hence the cigarette paper manufacturer has to be able to adjust the diffusion capacity of these areas with the most efficient means possible over the largest range possible, so that even smaller quantities of such papers can be produced cost-effectively. While a cigarette paper grade can in principle be used for many different cigarette constructions and thereby large production batches can be produced when there is no requirement for self-extinguishing, this requirement increases the number of different paper grades and consequently the size of a production batch decreases.
The prior art allows for several options to adjust the diffusion capacity of the treated areas of the cigarette paper.
One option consists in adjusting the geometry of the treated areas. The smaller the treated area, the less likely the self-extinguishing may be. A change of the geometry of the treated areas, however, is not particularly efficient, because it requires changing the printing cylinder, for example for printing processes such as roto-gravure printing, which takes some time and reduces productivity. In addition, for each geometry under consideration, one printing cylinder and potentially a reserve cylinder have to be procured and stored. Thus, this process is relatively expensive for small production batches.
A further option consists in varying the amount of composition applied to the cigarette paper per unit area of the treated areas. This can also be carried out for a printing process, for example, roto-gravure printing, by means of the printing cylinder. The printing cylinder has a plurality of small engraved or etched recesses corresponding to the printing pattern, into which the composition is picked up from a storage tank and is transferred from the recesses onto the paper. The applied amount can be influenced by the volume or other properties of the recesses. The disadvantages in relation to changing the printing cylinder and storing the printing cylinders are, however, the same as when adjusting the geometry of the areas.
Finally a further option consists in varying the proportion of the film-forming material in the composition. The less film-forming material is in the composition, the less film-forming material is transferred to the paper for the same applied amount of the composition. This method has the disadvantage that the viscosity of this composition also changes when the proportion of the film-forming material in the composition is changed. Most application processes do not permit large variations in the viscosity of the composition to be applied or require corresponding adjustments to the process parameters, such as velocities or drying temperatures, for which reason this method can be used only within tight limits. The viscosity can, however, also be adjusted by the selection of the film-forming material, but this is very tightly limited by the legal requirements regarding the components of cigarette paper as well as the influence on the taste of a cigarette manufactured from such a paper.
There is a need for an option for adjusting the diffusion capacity of the treated areas of the cigarette paper simply and inexpensively, so that comparatively small batches of cigarette paper can be produced efficiently each with a different diffusion capacity in the treated areas of the paper.